Non-LTR retrotransposons, including long interspersed elements type 1 (LINE-1s) and short interspersed elements, are highly abundant transposable elements in mammalian genomes. LINE-1s are autonomous: they not only replicate themselves but also mobilize Alus and SVAs. This process, termed retrotransposition, is inherently mutagenic and is responsible for >0.1% of spontaneous germline mutations in humans, including ~90 known cases of birth defects. Birth defects are the leading cause of infant death, but for the majority of birth defects the exact etiology is unknown, precluding effective preventive measures. Recent studies highlight the importance of environmental factors and epigenetic mechanisms, but how they lead to altered gene expression and function remains to be determined. Here, we propose to investigate an etiological role of LINE- 1 mediated insertional mutagenesis in major birth defects. We reason that such a role has been underappreciated because LINE-1 insertion polymorphisms are frequently missed by traditional genetic analyses. Indeed, a recent analysis of a cohort of unrelated neurofibromatosis type 1 patients, whose genetic alterations could not be explained by point mutations, revealed de novo insertions being the causative event for all 18 patients. The objective of this R21 application is o test the hypothesis that LINE-1 activities are developmentally regulated in the male germline, and that the frequency of retrotransposition increases in male germ cells with age. Mechanistically, a paternal age effect (PAE) may represent, at least in part, preferential retrotransposition in spermatogonia, or an age-related failure in the critical control mechanisms that regulate retrotransposons in spermatocytes, or both. In this proposal, we will use novel transgenic mouse models to specifically track LINE-1 mutagenesis burden during germ cell development, to determine whether de novo mutation by LINE-1 retrotransposition has a PAE, and to systematically profile LINE-1 expression pattern during mouse development. Our approach will generate novel data that are unattainable from human samples. It is expected to provide fundamental insights into mammalian genome biology and mutational mechanisms. It also represents a critical first step toward objective evaluation of the role of retrotransposon mutagens in sporadic human genetic disorders, with the potential to contribute to our understanding of mechanisms underlying PAEs on major birth defects.